Sheet paper feeder for two-sided recording

ABSTRACT

A paper feeder installed in a recording apparatus is capable of selectively performing two-sided recording on sheet paper and includes a paper supply passageway for supplying the sheet paper to a recording unit of the recording apparatus, a paper eject passageway for ejecting from the recording unit the sheet paper on which recording has been performed and a paper bypass passageway which extends between the paper supply passageway and the paper eject passageway. A paper switching unit is provided at a branched portion between the paper eject passageway and the paper bypass passageway. Paper feed rollers can be driven in forward and reverse directions so that the sheet paper can be fed along the paper eject passageway in two directions. The paper feed rollers are arranged on the downstream side of the paper switching unit in the ejection direction of the sheet paper. After the sheet paper on which recording has been performed on one side at the recording unit of the recording apparatus is once fed along the paper eject passageway by the forward direction driving operation of the paper feed rollers, the sheet paper is fed through the paper switching unit to the paper bypass passageway by the reverse direction driving operation of the paper feed rollers, whereby reversal of the sheet paper is carried out.

TECHNICAL FIELD

The present invention relates to a paper feeder built in a recordingapparatus such as a copier or printer, which paper feeder is constitutedso as to be able to enable two-sided recording selectively applied tosheet paper.

BACKGROUND ART

In general, a recording apparatus such as a copier or printer isprovided with a paper supply cassette holding a stack of sheet paper, arecording unit which performs recording on the sheet paper fed out ofthe paper supply cassette, and a paper receiver which receives the sheetpaper ejected from the recording unit. A paper supply passageway extendsbetween the paper supply cassette and the recording unit, and a papereject passageway extends between the recording unit and the paperreceiver. In short, the sheet paper to be recorded on is introduced fromthe paper supply cassette to the recording unit through the paper supplypassageway, recording is applied to one side of the sheet paper, andthis recorded sheet paper is fed to the paper receiver from therecording unit through the paper eject passageway.

Where two-sided recording is applied to sheet paper by such a recordingapparatus, after the recording is applied to one side of the sheetpaper, it is necessary to reverse that sheet paper and return it to therecording unit. For this reason, in a recording apparatus which canperform two-sided recording, a paper bypass passageway is providedbetween the paper supply passageway and the paper eject passageway, anda paper switching unit is installed at a branched portion of the papereject passageway and paper bypass passageway. During one-sidedrecording, the sheet paper is fed to the paper receiver through thepaper eject passageway, but during two-sided recording, the sheet paperis sent to the paper bypass passageway by the paper switching unit. Apaper reversal mechanism is installed in the paper bypass passageway,the sheet paper is reversed by the paper reversal mechanism and thenintroduced again into the recording unit, and thus the recording isapplied to the other side of the sheet paper. This sheet paper, that is,the sheet paper subjected to the two-sided recording, is fed from therecording unit to the paper receiver through the eject passageway.

A typical conventional paper reversal mechanism contains a paperreversal and accommodating portion provided midway of the paper bypasspassageway, which paper reversal and accommodating portion divides thepaper bypass passageway into an upstream part and a downstream part. Thepaper reversal mechanism further contains a roller assembly installed inthe paper reversal and accommodating portion, which roller assemblycontains an intermediate roller and two side rollers engaged with thisintermediate roller. The intermediate roller is arranged at the branchedportion of the upstream part and downstream part of the paper bypasspassageway. At this time, one of the two side rollers, that is, a firstside roller, is positioned in the upstream part of the paper bypasspassageway, and the other side roller, that is, a second side roller, ispositioned in the downstream part of the paper bypass passageway. Thesheet paper sent through the upstream part of the paper bypasspassageway is pulled into the paper reversal and accommodating portionby the first side roller and intermediate roller. When the rear edgethereof leaves the nip between the first side roller and intermediateroller, the rear edge is grasped by the nip between the second sideroller and intermediate roller, whereby the sheet paper is sent to thedownstream part of the paper bypass passageway by the second side rollerand intermediate roller. Thus, a reversal of the sheet paper isobtained, whereby two-sided recording on the sheet paper becomespossible.

As apparent from the above description, in the conventional paperreversal mechanism, at the time of reversal of the sheet paper, thesheet paper must be completely accommodated in the paper reversal andaccommodating portion. In other words, the length of the paper reversaland accommodating portion must correspond to the length of the largestsize of sheet paper. For this reason, a recording apparatus having apaper reversal mechanism as mentioned above is enlarged in size due tothe paper reversal and accommodating portion thereof.

DISCLOSURE OF THE INVENTION

Accordingly, a main object of the present invention is to provide apaper feeder which is built into a recording apparatus such as a copieror printer and enables selective application of two-sided recording tosheet paper, which paper feeder is constituted so that it can contributeto the reduction of size of the recording apparatus.

Another object of the present invention is to provide a paper feeder asmentioned above which is constituted so that the recording with respectto the sheet paper can be efficiently carried out.

The paper feeder according to the present invention is used for enablingselective performance of two-sided recording on sheet paper by arecording apparatus such as a copier or printer and is provided with apaper supply passageway means for supplying the sheet paper to therecording unit of the recording apparatus; a paper eject passagewaymeans for ejecting from the recording unit the sheet paper on whichrecording was performed at the recording unit of the recordingapparatus; and a paper bypass passageway means which extends between thepaper supply passageway means and the paper eject passageway means.According to the present invention, a paper switching means is providedat a branched portion between the paper eject passageway means and thepaper bypass passageway means, and a paper feed roller means which canreverse the paper sheet so that the sheet paper can be fed in twodirections along the paper eject passageway means is provided in thepaper eject passageway means. The paper feed roller means is arranged onthe downstream side of the paper switching means in the ejectiondirection of the sheet paper. At the time of two-sided recording, thesheet paper on which recording was performed on one side at therecording unit of the recording apparatus is once fed along the papereject passageway means by the forward direction driving operation of thepaper feed roller means and is then fed to the paper bypass passagewaymeans through the paper switching means by the reverse direction drivingoperation of the paper feed roller means, whereby the reversal of thesheet paper is performed.

In the paper feeder according to the present invention, preferably thecontrol of the change-over of the paper feed roller means from theforward direction driving operation to the reverse direction drivingoperation is carried out by the sheet paper detection means installed inan appropriate portion of the paper eject passageway means based on thedetection of the passing of the sheet paper at that portion.

Also, preferably, at the time of two-sided recording, the forwarddirection driving operation of the paper feed roller means is controlledso that the feeding speed of the sheet paper becomes higher than theusual feeding speed thereof over at least a part of a period where thesheet paper on which recording was performed on one side at therecording unit of the recording apparatus is once fed along the papereject passageway means by the forward direction driving operation of thepaper feed roller means. Further, the reverse direction drivingoperation of the paper feed roller means is controlled so that thefeeding speed of the sheet paper becomes higher than the usual feedingspeed thereof over at least a part of a period where the sheet paper isfed to the paper bypass passageway means through the paper switchingmeans by the reverse direction driving operation of the paper feedroller means.

The paper switching means can be constituted as a roller assembly. Inthis case, that roller assembly preferably comprises a blade-equippedroller element arranged at the branched portion between the paper ejectpassageway means and the paper bypass passageway means; and two sideroller elements which are engaged with this blade-equipped rollerelement and arranged on the respective sides of the paper ejectpassageway means and paper bypass passageway means. At this time, thedirection of rotation of the blade-equipped roller element is reversedwith respect to the direction of rotation of the two side rollerelements, but the circumferential speed of these three roller elementsis made substantially equal to the usual feeding speed of the sheetpaper. On the other hand, the paper switching means can be constitutedalso only by a blade-equipped roller element arranged at the branchedportion between the paper eject passageway means and paper bypasspassageway means. In this case, the circumferential speed of theblade-equipped roller element can be made larger than the usual feedingspeed of the sheet paper.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and other advantages of the present invention will beclarified by the following description with reference to attacheddrawings wherein:

FIG. 1 is a schematic view of a laser printer in which is built a firstembodiment of a paper feeder according to the present invention;

FIG. 2 is a partial enlarged view of the laser printer shown in FIG. 1;

FIG. 3 is a partial enlarged view of the paper feeder shown in FIG. 2;

FIG. 4 is a control block diagram of the paper feeder shown in FIG. 2;

FIGS. 5A and 5B are a flow chart explaining the actuation of the paperfeeder shown in FIG. 2;

FIG. 6 is a timing chart in relation to the flow chart shown in FIGS. 5Aand 5B;

FIG. 7 is another timing chart in relation to the flow chart shown inFIGS. 5A and 5B;

FIG. 8 is a schematic view of a laser printer in which is built a secondembodiment of the paper feeder according to the present invention;

FIG. 9 is a partial enlarged view of the laser printer shown in FIG. 8;

FIG. 10 is a partial enlarged view of the paper feeder shown in FIG. 9;

FIGS. 11, 11(A), 11(B) and 11(C) are a flow chart explaining theactuation of the paper feeder shown in FIG. 9;

FIG. 12 is a timing chart in relation to the flowchart shown in FIGS.11(A) to 11(C);

FIGS. 13(a) to 13(d) are an explanatory views used for an explanation ofthe flow chart shown in FIGS. 11(A) to 11(C);

FIG. 14 is a partial enlarged view corresponding to FIG. 10 and is aview indicating a modified embodiment of the second embodiment of thepaper feeder according to the present invention; and

FIG. 15 is a partial enlarged view corresponding to FIG. 10 and is aview indicating another modified embodiment of the second embodiment ofthe paper feeder according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a laser printer including a paper feeder accordingto the present invention is schematically shown. The laser printer isprovided with a housing 10. A paper supply cassette 12 accommodating thestack of the sheet paper is provided at the bottom portion of thishousing 10. Also, in the housing 10, a recording unit, that is, aprinting unit 14, is arranged above the paper supply cassette 12. Thepaper receiver 16 is formed at the top portion of the housing 10. Thepaper feeder according to the present invention is indicated overall bythe reference numeral 18. This paper feeder 18 is provided with a papersupply passageway 20 which extends between the paper supply cassette 12and the printing unit 14; a paper eject passageway 22 which extendsbetween the printing unit 14 and the paper receiver 16; a paper bypasspassageway 24 which extends between the paper supply passageway 20 andthe paper eject passageway 22; and a paper switching means 26 arrangedat the branched portion between the paper eject passageway 22 and thepaper bypass passageway 24. Note that, the paper supply passageway 20,the paper eject passageway 22, and the paper bypass passageway 24 areformed by appropriately arranging the guide plate elements.

The paper supply cassette 12 is provided with a feed out roller 12a. Thepaper sheets are fed out one by one from the stack thereof by this feedout roller 12a. Three pairs of paper feed rollers 28a and 28b, 30a and30b, and 32a and 32b are installed in the paper supply passageway 20 atappropriate intervals. The sheet paper fed out from the paper supplycassette 12 is fed toward the printing unit 14 by three pairs of paperfeed rollers, but when the leading edge of the sheet paper reaches onepair of register rollers 34a and 34b, the sheet paper is temporarilystopped.

As shown in FIG. 1 and FIG. 2, the printing unit 14 is provided with aphotosensitive drum 14a. This photosensitive drum is rotated in aclockwise direction at the actuation of the laser printer as indicatedby an arrow in FIG. 1. The photosensitive drum 14a is formed by forminga photoconductive material layer, that is, a photosensitive materialfilm layer, on the surface of a cylindrical base made of, for example,aluminum. As such a photosensitive material, for example, an organicphotosensitive material, a selenium-based photosensitive material, anamorphous silicon photosensitive material, etc. have been known.Electric charges are given to the photosensitive drum 14a by anappropriate electric charger, for example a corona charger 14b, wherebya uniform charged region is formed in the photosensitive material filmlayer thereof. An electrostatic latent image is written in the chargedregion of the photosensitive drum 14a by a laser beam scanning unit 14c.The writing of this electrostatic latent image is carried out byrepeatedly scanning the laser beam LB emitted from the laser beamscanning unit 14d along a longitudinal direction of the photosensitivedrum 14a and, at the same time, turning on and off the laser beam LBbased on binary image data from, for example, a word processor or amicrocomputer. The electrostatic latent image written in this way iselectrostatically developed as a charged toner image by the developer14d.

The charged toner image is moved toward an appropriate electric charger,for example, a corona charger 14e, arranged on the bottom thereof by therotation of the photosensitive drum 14a. On the other hand, a pair ofregister rollers 34a and 34b are driven at a predetermined timing tointroduce the sheet paper into a gap between the photosensitive drum 14aand the corona charger 14e at the same speed as the circumferentialspeed of the photosensitive drum 14a. At this time, the corona charger14e gives electric charges having a reverse polarity to that of thecharged toner image to the sheet paper, whereby the charged toner imageis electrostatically transferred from the photosensitive drum 14a to thesheet paper. As mentioned above, a pair of register rollers 34a and 34bare driven at a predetermined timing, and therefore the transfer of thecharged toner image is carried out at a proper position with respect tothe sheet paper. An AC discharger 14f is arranged adjacent to the coronacharger 14e. This AC discharger 14f removes a part of the electriccharges from the sheet paper. For this reason, the electrostaticallyattraction force acting upon a space between the sheet paper and thephotosensitive drum 14a is weakened, and thus the winding of the sheetpaper around the photosensitive drum 14a can be prevented. Note that, inFIG. 1 and FIG. 2, reference numeral 14g indicates a toner cleaner. Theresidual toner remaining on the photosensitive drum 14a without transferto the sheet paper from the photosensitive drum 14a is removed by thistoner cleaner 14g.

As clear from the above description, the circumferential speed of thephotosensitive drum 14a regulates the printing speed of the laserprinter, that is, the feeding speed of the sheet paper. In the presentembodiment, the circumferential speed of the photosensitive drum 14a isset to 133 mm/sec.

A heat fixing unit 36 is installed in the paper eject passageway 22. Thesheet paper ejected from a space between the photosensitive drum 14a andAC discharger 14f is immediately sent to the heat fixing unit 36, atwhich the transferred toner image is heat fixed on the sheet paper.Namely, the heat fixing unit 36 comprises a heat roller 36a and a backuproller 36b. When the sheet paper is passed between them, the transferredtoner image is thermally melted and fixed on the sheet paper. Note that,the circumferential speed of the heat roller 36a and the backup roller36b is set to the same speed as the circumferential speed of thephotosensitive drum 14a (that is, the feeding speed of the sheet paper).

In the present embodiment, the paper switching means 26 is constitutedas a roller assembly comprising three rollers. Namely, the paperswitching unit 26 includes the intermediate roller 26a arranged at thebranched portion between the paper eject passageway 22 and the paperbypass passageway 24. This intermediate roller 26a is preferably formedas a blade-equipped roller. The paper switching means 26 furtherincludes the two side rollers 26b and 26c engaged with the intermediateroller, that is, the blade-equipped roller 26a. One side roller, thatis, the first side signal roller 26b, is installed in the paper ejectpassageway 22, and the other side roller, that is, the second sideroller 26c, is installed in the paper bypass passageway 24. Theblade-equipped roller 26a is formed by embedding a large number ofblades in the rotation shaft thereof in the radial direction. Each bladeis formed by an appropriate rubber material or a resin material. Theblade-equipped roller 26a is rotated in the counterclockwise directionas indicated by an arrow in FIG. 3, and the circumferential speedthereof is made the same as the circumferential speed of thephotosensitive drum 14a (feeding speed of the sheet paper). On the otherhand, the first side roller 26b and the second side roller 26c arerotated in the clockwise direction as indicated by the arrows in FIG. 3,and the circumferential speed thereof is equalized to that of theblade-equipped roller 26a. Note that, during the actuation of the laserprinter, the three rollers 26a, 26b, and 26c of the paper switchingmeans 26 are always rotated by a main motor (not illustrated) forrotating the photosensitive drum 14a of the printing unit 14 and tworollers 36a and 36b of the heat fixing unit 36 etc.

Three pairs of paper feed rollers 38a and 38b, 40a and 40b, and 42a and42b are installed in the paper eject passageway 22 at appropriateintervals. The rollers 38a, 40a, and 42a of the pairs of the paper feedrollers are used as the drive rollers, and the other paper feed rollers30b, 40b, and 42b are used as the driven rollers. The drive rollers 38a,40a, and 42a are simultaneously rotated by the same drive source, forexample, a step motor (not illustrated in FIG. 1 and FIG. 2) at the samecircumferential speed as the circumferential speed of the photosensitivedrum 14a (feeding speed of the sheet paper). The driving operation ofthe step motor can be reversed. Namely, the drive roller 38a (40a, 42a)may be rotated in both of the clockwise direction and counterclockwisedirection as indicated by the two arrows in FIG. 3. Here, forconvenience of the latter explanation, when the abovementioned stepmotor is driven to rotate the drive roller 38a (40a, 42a) in theclockwise direction, that driving direction is defined as the forwarddirection, and when the abovementioned step motor is driven to rotatethe drive roller 38a (40a, 42a) in the center clockwise direction, thatdriving direction is defined as the reverse direction. Note that, a pairof paper eject rollers 44a and 44b are provided at the outlet end of thepaper eject passageway 22.

As shown in FIG. 1, two pairs of paper feed rollers 46a and 46b and 48aand 48b are installed also in the paper bypass passageway 24 atappropriate intervals. The rollers 46a and 48a in the pairs of paperfeed rollers are used as the drive rollers, and the other paper feedrollers 46b and 48b are used as the driven rollers. The drive rollers46a and 48a are always rotated in only one direction by the same drivesource, for example, a step motor (not illustrated in FIG. 1 and FIG. 2)at the circumferential speed of the photosensitive drum 14a (feedingspeed of the sheet paper). Namely, according to the abovementioneddefinition, the drive rollers 46a and 48a are always rotated only in thereverse direction (counterclockwise direction).

As shown in FIG. 1, an appropriate paper detector 50 is installed in thepaper eject passageway 22. This paper detector 50 is arranged on theheat fixing unit 36 side close to the first side roller 26b of the paperswitching means 26. By such a paper detector 50, the leading edge andtrailing edge of the sheet paper ejected from the heat fixing unit 36are detected. As clear from FIG. 3, in the present embodiment, a leveractuation type microswitch is used as the paper detector 50. Anothertype of detector also, for example, an optical sensor can be used as thepaper detector 50.

The paper feeder according to the present invention is provided with acontrol circuit 52 as shown in FIG. 4. This control circuit isconstituted by a microcomputer. As illustrated, the microcomputerincludes a central processing unit (CPU) 52a, a read only memory (ROM)52b storing an actuation program, constants, etc., a random accessmemory (RAM) 52c storing temporary data etc., and an input/outputinterface (I/O) 52d.

The paper detector, that is, the microswitch 50, is connected via an A/Dconverter 54 to the I/O 52d of the control circuit 52. When the lever ofthe microswitch 50 is pushed in contact with the sheet paper, the outputsignal from the A/D converter 54 is brought to a low level "L", and in acase other than this, the output signal from the A/D converter 54 isbrought to a high level "H". In short, at the passing of the sheet paperat the position of deployment of the microswitch 50, when the leadingedge of the sheet paper comes into contact with the lever of themicroswitch 50, the output signal from the A/D converter 54 is changedover from the high level "H" to the low level "L", and when the trailingedge of the sheet paper leaves the lever of the microswitch 50, theoutput signal from the A/D converter 54 is changed over from the lowlevel "L".

The step motor SM1 is the motor for driving the three drive rollers 38a,40a, and 42a installed in the paper eject passageway 22. These driverollers are simultaneously rotated by the step motor SM1 via anappropriate drive transfer means, for example, a belt or a gear train.Also, the step motor SM2 is the motor for driving the drive rollers 46aand 48a installed in the paper bypass passageway 24. These drive rollersare also simultaneously rotated by the step motor SM2 via an appropriatedrive transfer means. The step motors SM1 and SM2 are connected via thedrive circuits D1 and D2, respectively, to the I/O 52d. The respectivedrive circuits D1 and D2 are controlled by the control signal preparedby the control circuit, that is, the microcomputer 52, whereby the drivepulse is output from the respective drive circuits D1 and D2 to therelated step motors SM1 and SM2. Note that, it is well known to have theturning on/off, acceleration, deceleration, and reverse driving of thestep motor controlled by a microcomputer.

FIGS. 5A and 5B show a routine for actuating the paper feeder accordingto the present invention; and FIG. 6 and FIG. 7 indicate timing chartsin relation to the routine of FIGS. 5A and 5B. This routine is startedby turning on the power switch 56 (FIG. 4) of the laser printer and isexecuted by an interruption signal output at a predetermined timeinterval, for example, at every 1 ms.

At step 501, it is decided whether or not the flag F₁ is "0". In aninitial state, F₁ =0, and therefore the routine goes to step 502, atwhich it is decided whether or not the flag BF is "0". The flag BFindicates whether one-sided printing should be carried out by the laserprinter or whether two-sided printing should be carried out, and thewriting of "0" or "1" to the flag BF is carried out by an instructionfrom a word processor or a personal computer connected to the laserprinter. Namely, when BF=0, the one-sided printing is carried out, andwhen BF=1, the two-sided printing is carried out.

When the one-sided printing is carried out, that is, when BF=0 , theroutine goes from step 502 to step 503. At step 503, it is decidedwhether or not the flag F₂ is "0". In the initial state, F₂ =0, andtherefore the routine goes to step 504, at which it is decided whetherthe output signal from the paper detector 50 (that is, the A/D converter54) is the low level "L" or high level "H" (FIG. 6). When the outputsignal from the paper detector 50 is at the high level "H", that is,where the leading edge of the sheet paper ejected from the heat fixingunit 36 has not yet been detected by the paper detector 50, the routineis once ended.

After an elapse of 1 ms, the routine is executed again, but no progressis made until the leading edge of the sheet paper ejected from the heatfixing unit 36 is detected by the paper detector 50. Namely, at step504, the detection of the leading edge of the sheet paper by the paperdetector 50 is monitored.

When the output signal from the paper detector 50 is changed over fromthe high level "H" to the low level "L", that is, when the leading edgeof the sheet paper ejected from the heat fixing unit 36 is detected bythe paper detector 50, the routine goes from step 504 to 505, at whichit is decided whether or not the time t₁ has elapsed. No progress ismade so far as the time t₁ has not elapsed. The time t₁ is a timerequired for the leading edge of the sheet paper ejected from the heatfixing unit 36 to pass between the blade-equipped roller 26a of thepaper switching unit 26 and the first side roller 26b and to reach apair of paper feed rollers 38a and 38b from a point of time when it isdetected by the paper detector 50. Note that, in the present embodiment,the time t₁ is 400 ms.

When the time t₁ has elapsed, the routine goes from step 505 to 506, atwhich the step motor SM1 is accelerated to a predetermined speed in theforward direction. At this time, the drive rollers 38a, 40a, and 42a arerotated at a circumferential speed of 133 mm/sec in the forwarddirection (clockwise direction), whereby the sheet paper is fed alongthe paper eject passageway 22 toward the outlet end thereof. Note that,as shown in FIG. 6, the acceleration of the step motor SM1 to thepredetermined speed is carried out over a time of for example 40 ms.

At step 507, the flag F₂ is rewritten from "0" to "1", and subsequentlythe routine goes to step 508, at which it is decided whether the outputsignal from the paper detector 50 is the low level "L" or high level"H". Namely, at step 508, it is monitored whether or not the trailingedge of the sheet paper is detected by the paper detector 50.

At step 508, when the trailing edge of the sheet paper is detected bythe paper detector 50, that is, when the output signal from the paperdetector 50 is changed over from the low level "L" to the high level"H", the routine goes from step 508 to step 509, at which it is decidedwhether or not the time t₂ has elapsed (FIG. 6). The time t₂ is a timerequired for the trailing edge of the sheet paper to leave the pair ofpaper feed rollers 42a and 42b from a point of time when it is detectedby the paper detector 50.

At step 509, when the time t₂ has elapsed, the routine goes to step 510,at which the drive of the step motor SM1 is decelerated and stopped(FIG. 6). The sheet paper leaving the pair of the paper feed rollers 42aand 42b is ejected onto the paper receiver 16 by a pair of paper ejectrollers 44a and 44b. Note that, the deceleration time of the step motorSM1 is 40 ms, which is the same as the abovementioned acceleration time.

Subsequently, the routine goes to step 511, at which the flag F₂ isrewritten from "1" to "0", and then it is decided at step 512 whether ornot the flag F₃ is "0". At the present time, F₂ =0, and therefore theroutine is once ended.

The above explained actuation is for performing the one-sided printingby a laser printer and is repeated whenever the sheet paper is ejectedfrom the heat fixing unit 36.

Where the two-sided printing is carried out by the laser printer, thatis, where "1" is written in the flag BF, the routine goes from step 502to step 513, at which it is decided whether or not the flag F₂ is "0".At this point of time, F₂ =0, and therefore the routine goes from step513 to step 514, at which it is decided whether the output signal fromthe paper detector 50 is the low level "L" or the high level "H". Whenthe output signal from the paper detector 50 is at the high level "H",that is, when the leading edge of the sheet paper ejected from the heatfixing unit 36 has not yet been detected by the paper detector 50, theroutine is once ended.

After an elapse of 1 ms, the routine is executed again, but no progressis made until the leading edge of the sheet paper ejected from the heatfixing unit 36 is detected by the paper detector 50. Namely, at step514, the detection of the leading edge of the sheet paper by the paperdetector 50 is monitored.

When the output signal of the paper detector 50 is changed over from thehigh level "H" to the low level "L" (FIG. 7), that is, when the leadingedge of the sheet paper ejected from the heat fixing unit 36 is detectedby the paper detector 50, the routine goes from step 514 to 515, atwhich it is decided whether or not the time T₁ has elapsed. No progressis made so long as the time T₁ has not elapsed. The time T₁ is a timerequired for the leading edge of the sheet paper ejected from the heatfixing unit 36 to pass between the blade-equipped roller 26a and thefirst side roller 26b and to reach a pair of paper feed rollers 38a and38b from a point of time when it is detected by the paper detector 50.Namely, the time T₁ is equal to the abovementioned time t₁ (400 ms).

When the time T₁ has elapsed, the routine goes from step 515 to 516, atwhich the step motor SM1 is accelerated to the predetermined speed inthe forward direction (FIG. 7). At this time, the drive rollers 38a,40a, and 42a are rotated at the circumferential speed of 133 mm/sec inthe forward direction (clockwise direction), whereby the sheet paper isfed along the paper eject passageway 22 toward the outlet end thereof.Note that, as clear from FIG. 7, the acceleration time to thepredetermined speed of the step motor SM1 is 40 ms.

At step 517, the flag F₂ is rewritten from "0" to "1", and subsequentlythe routine goes to step 518, at which it is decided whether the outputsignal from the paper detector 50 is the low level "L" or the high level"H". Namely, at step 518, it is monitored whether or not the trailingedge of the sheet paper is detected by the paper detector 50.

At step 518, when the trailing edge of the sheet paper is detected bythe paper detector 50, that is, when the output signal from the paperdetector 50 is changed over from the low level "L" to the high level"H", the routine goes to step 519, at which the driving operation of thestep motor SM1 is decelerated and stopped. The deceleration time of thestep motor SM1 is 40 ms as is apparent from FIG. 7. At this time, thetrailing edge of the sheet paper stops at a position away from thedetection portion by the paper detector 50 by only 5.4 mm. As clear fromFIG. 3, in the present embodiment, the outer diameter of theblade-equipped roller 26a is 12 mm, and the horizontal distance from thedetection portion by the paper detector 50 up to the vertical axial linepassing through the center of the blade-equipped roller 26a is 2.4 mm.For this reason, the trailing edge of the sheet paper will stop at theposition away from the vertical axial line passing through the center ofthe blade-equipped roller 26a by 3 mm on the eject direction side. Notethat, in FIG. 3, "r" indicates the radius of the blade-equipped roller26a.

Subsequently, when the routine goes to step 520, it is decided whetheror not the time T₂ has elapsed. No progress is made so long as the timeT₂ has not elapsed. The time T₂ is appropriately selected and set to atime within a range of from, for example, 59 through 270 ms. In short,the sheet paper is stopped only during a period of time T₂. At thistime, the trailing edge of the sheet paper is directed to the paperbypass passageway 24 side by the blade-equipped roller 26a.

At step 520, when the time T₂ has elapsed, the routine goes to step 521,at which the step motor SM1 is accelerated to the predetermined speed ina reverse direction, whereby the sheet paper is fed toward the paperbypass passageway 24. In this case, the circumferential speed of thedrive rollers 38a, 40a, and 42a is set to 133 mm/sec, and therefore thesheet paper is made to smoothly pass between the blade-equipped roller26a and the second side roller 26c. Note that, the acceleration time ofthe step motor SM1 is 40 ms.

Subsequently, when the routine goes to step 522, it is decided whetheror not the time T₃ has elapsed. No progress is made so long as the timeT₃ has not elapsed. The time T₃ is a time required for the leading edgeof the sheet paper to reach just before the paper feed rollers 46a and46b provided in the paper bypass passageway 24 from a point of time whenit is fed toward the paper bypass passageway 24.

At step 522, when the time T₃ has elapsed, the routine goes to step 523,at which the step motor SM2 is accelerated to the predetermined speed inthe reverse direction, whereby the sheet paper is fed along the paperbypass passageway 24 toward a pair of register rollers 34a and 34b. Notethat, the acceleration time of the step motor SM2 is 40 ms, and thecircumferential speed of the drive rollers 46a and 48b is set to 133mm/sec.

Subsequently, at step 524, it is decided whether or not the time T₄ haselapsed. No progress is made so long as the time T₄ has not elapsed. Thetime T₄ is a time required for the trailing edge of the sheet paper toleave from a pair of paper feed rollers 38a and 38b from a point of timewhen the step motor SM2 is accelerated.

At step 524, when the time T₄ has elapsed, the routine goes to step 525,at which the step motor SM1 is decelerated and stopped. Note that, thedeceleration time of the step motor SM1 is 40 ms.

Subsequently, at step 526, it is decided whether or not the time T₅ haselapsed. No progress is made so long as the time T₅ has not elapsed. Thetime T₅ is a time required for the leading edge of the sheet paper toreach just before a pair of register rollers 34a and 34b from a point oftime when the step motor SM1 is decelerated.

At step 526, when the time T₅ has elapsed, the routine goes to step 527,at which the step motor SM2 is decelerated and stopped. Note that, thedeceleration time is 40 ms.

Subsequently, at step 528, it is decided whether or not the time T₆ haselapsed. No progress is made so long as the time T₆ has not elapsed. Thetime T₆ is a time required for the leading edge of the sheet paper to beintroduced into the recording unit 14 by a pair of register rollers 34aand 34b from a point of time when the step motor SM2 is decelerated.

At step 528, when the time T₆ has elapsed, the routine goes to step 529,at which the step motor SM2 is accelerated in a reverse direction. Notethat, the acceleration time of the step motor SM2 is 40 ms.

Subsequently, at step 530, it is decided whether or not the time T₇ haselapsed. No progress is made so long as the time T₇ has not elapsed. Thetime T₇ is a time required for the trailing edge of the sheet paper toleave from a pair of paper feed rollers 48a and 48b from a point of timewhen the step motor SM2 is accelerated at step 529.

At step 528, when the time T₇ has elapsed, the routine goes to step 531,at which the step motor SM2 is decelerated and stopped. Note that, thedeceleration time is 40 ms.

In FIG. 2, reference symbol P1 indicates a sheet paper initiallyintroduced into the recording unit 14 at the time of two-sided printing.The printing is applied to only one side of this sheet paper P1.Accordingly, so as to apply the printing to the other surface of thesheet paper P1, the sheet paper P1 must be fed along the paper bypasspassageway 24 toward a pair of register rollers 34a and 34b as mentionedabove. Note that, in FIG. 2, the sheet paper P1 fed along the paperbypass passageway 24 is indicated by a one dot chain line. In thepresent embodiment, due to an increase of the amount of the printingprocessing at the laser printer, during a period where the sheet paperP1 is fed from the paper eject passageway 22 toward the paper bypasspassageway 24, a second sheet paper P2 has been already introduced intothe recording unit 14, and the printing is applied to the other surfaceof the sheet paper P2 after the printing is applied to one side of thesheet paper P2. For this reason, in the present embodiment, the printinginitially applied to the one side of the sheet paper P1 is carried outbased on either of the printing data of first page and second page amongthe printing data held in the word processor or personal computer (forexample the printing data of the second page), while the printinginitially applied to one side of the sheet paper P2 is carried out basedon either of the printing data of the third page and fourth page amongthe printing data (for example the printing data of the fourth page).Subsequently, where the printing is applied to the other surface of thesheet paper P1, the printing data of the first page is used, while wherethe printing is applied to the other surface of the sheet paper P2, theprinting data of the third page is used. Where the printing is appliedto the papers P1 and P2 in such a manner, the sheet papers P1 and P2 areejected onto the paper receiving holder 16 in a proper order of pages.Note that, a printing mode as mentioned above is disclosed in detail inJapanese Unexamined Patent Publication (Kokai) No. 2-39966.

An explanation will be made again of the routine shown in FIG. 5.

After the step motor SM2 is decelerated at step 531, the routine goes tostep 532, at which the flag F₂ is rewritten from "1" to "0".Subsequently, at step 533, the count value of the counter C is countedup exactly by +1. At step 534, it is decided whether or not the countvalue of the counter C is equal to "2". When C is not equal to 2, theroutine is once ended.

Thereafter, when the second order sheet paper (P2) is returned to a pairof register rollers 34a and 34b for the two-sided printing in the samemode, since the value of the counter C has been brought to "2", theroutine goes to step 535, at which the flag F₁ is rewritten from "0" to"1", and at step 536, also the flag F₃ is rewritten from "0" to "1".Subsequently, after the counter C is reset at step 537, it is returnedto step 501.

At this time, F₁ =1, and therefore the routine goes from step 501 tostep 503, at which the above-mentioned sheet paper ejection actuation(step 504 through 512) is carried out, whereby an initial sheet paper(P1), that is, the sheet paper subjected to the two-sided printing, isejected onto the paper receiving holder 16. On the other hand, the flagF₃ =1 at this time, and therefore the routine goes from step 512 to step538, at which the count value of the counter C is counted up exactly by+1. At step 539, it is decided whether or not the count value of thecounter C is equal to "2". When C is not equal to "2", the routine isonce ended.

Subsequently, a similar paper ejection actuation is repeated (step 504through 512) for ejecting the second sheet paper (P2) onto the paperreceiving holder 16. At this time, the value of the counter C has beenbrought to "2", and therefore the routine goes to step 540, at which theflag F₁ is rewritten from "1" to "0", and at step 541, also the flag F₃is rewritten from "1" to "0". Subsequently, after the counter C is resetat step 542, the routine is once ended.

Further, when the two-sided printing is carried out also with respect tothe third and fourth sheet paper, the feeding of these sheet papers iscarried out by the same mode.

As apparent from the above description, according to the presentinvention, the paper eject passageway per se is utilized as the paperreversal and accommodating unit for reversing the sheet paper at thetime of two-sided printing, and therefore it becomes unnecessary toprovide such a paper reversal and accommodating unit in the paper bypasspassageway. Accordingly, the paper feeder according to the presentinvention can contribute to the reduction of size of a recordingapparatus such as a copier or printer constituted so as to be able toperform the two-sided recording.

FIG. 8, FIG. 9, and FIG. 10 indicate a second embodiment of a paperfeeder according to the present invention. Note that, in FIG. 8, FIG. 9,and FIG. 10, the same constituent elements as the constituent elementsof the above-mentioned first embodiment are indicated by the samereference numerals. In the second embodiment, the paper switching means26 comprises only the blade-equipped roller 26a. This blade-equippedroller 26a is rotated in the counterclockwise direction so that thecircumferential speed thereof becomes faster than the usual feedingspeed 133 mm/sec, for example, 672 mm/sec. Note that, during theactuation of the laser printer, the blade-equipped roller 26a iscontinuously being rotated. Also, in the second embodiment, in additionto the two pairs of the paper feed rollers 46a and 46b and rollers 48aand 48b, another pair of paper feed rollers 58a and 58b are installed inthe paper bypass passageway 24, and in addition, arranged close to theblade-equipped roller 26a. The paper feed roller 58a is formed as thedrive roller and is driven in the same way as the drive rollers 46a and48a by the step motor SM2. Note that, the paper feed roller 58b isformed as the driven roller.

FIGS. 11(A)-11(C) shown a routine for actuating the paper feeder of thesecond embodiment; and FIG. 12 is a timing chart in relation to theroutine of FIGS. 11(A)-11(C). In the same way as the routine shown inFIG. 5, also the routine of FIGS. 11(A)-11(C) is activated by turning onthe power source switch 56 (FIG. 4) and is executed by the interruptionsignal output at a predetermined time interval, for example, at every 1ms.

At step 1101, it is decided whether or not the flag F₁ is "0". In theinitial state, F₁ =0, and therefore the routine goes to step 1102, atwhich it is decided whether or not the flag BF is "0". In the same wayas the case of the routine shown in FIG. 5, the flag BF indicateswhether the one-sided printing should be carried out by the laserprinter, or the two-sided printing should be carried out thereby, andthe writing of "0" or "1" to the flag BF is carried out by theinstruction from the word processor or personal computer connected tothe laser printer. Namely, when BF=0, the one-sided printing is carriedout, while when BF=1, the two-sided printing is carried out.

When the one-sided printing is carried out, the mode of feeding of thesheet paper is the same as the case of the routine of FIG. 5, and thesheet paper is ejected onto the paper receiving holder 16 according tothe timing chart of FIG. 6. In short, steps 1101 through 1112substantially coincide with steps 501 through 512 of FIG. 5.

Where the two-sided printing is carried out, that is, where "1" has beenwritten in the flag BF, the routine goes from step 1102 to step 1113, atwhich it is decided whether or not the flag F₂ is "0". In the initialstate, since F₂ =0, the routine goes from step 1113 to step 1114, atwhich it is decided whether the output signal from the paper detector 50is the low level "L" or the high level "H". When the output signal fromthe paper detector 50 is at the high level "H", that is, when theleading edge of the sheet paper ejected from the heat fixing unit 36 hasnot yet been detected by the paper detector 50, the routine is onceended.

After an elapse of 1 ms, the routine is executed again, but no progressis made until the leading edge of the sheet paper ejected from the heatfixing unit 36 is detected by the paper detector 50. Namely, at step1114, the detection of leading edge of the sheet paper by the paperdetector 50 is monitored.

When the output signal of the paper detector 50 is changed over from thehigh level "H" to low level "L" (FIG. 12), that is, when the leadingedge of the sheet paper ejected from the heat fixing unit 36 is detectedby the paper detector 50, the routine goes from step 1114 to 1115, atwhich it is decided whether or not the time T₁ has elapsed. No progressis made so long as the time T₁ has not elapsed. The time T₁ is a timerequired for the leading edge of the sheet paper ejected from the heatfixing unit 36 to pass the blade-equipped roller 26a and reach a pair ofpaper feed rollers 38a and 38b from when it is detected by the paperdetector 50. Namely, the time T₁ is equal to the time t₁ mentionedpreviously (400 ms).

After an elapse of the time T₁, the routine goes from step 1115 to 1116,at which the step motor SM1 is accelerated to the first speed in theforward direction (FIG. 12). In this case, the drive rollers 38a, 40a,and 42a are rotated in the forward direction (clockwise direction) atthe circumferential speed of 133 mm/sec, whereby the sheet paper is fedalong the paper eject passageway 22 toward the outlet end thereof (FIG.13(a)). The circumferential speed 133 mm/sec coincides with the usualfeeding speed of the sheet paper in the same way as the case of theabove-mentioned embodiment. Also, as clear from FIG. 12, theacceleration time of the step motor SM1 to the first speed is 40 ms.

At step 1117, it is decided whether or not the time T₂ has elapsed. Noprogress is made so long as the time T₂ has not elapsed. The time T₂ isa time required for the trailing edge of the sheet paper to leave fromthe heat fixing unit 36 from a point of time when the step motor SM1starts to be accelerated to the first speed. Note that, in the presentembodiment, the time T₂ is set to 700 ms.

At step 1117, when the time T₂ has elapsed, that is when the trailingedge of the sheet paper leaves the heat fixing unit 36, the routine goesto step 1118, at which the step motor SM 1 is accelerated to the secondspeed in the forward direction. At this time, the circumferential speedof the drive rollers 38a, 40a, and 42a is accelerated from 133 mm/sec tothe circumferential speed of the blade-equipped roller 26a, i.e., 672mm/sec, and therefore the sheet paper is fed along the paper ejectpassageway 22 at a high speed of 672 mm/sec without receiving resistancefrom the blade-equipped roller 26a toward the outlet end thereof. Notethat, the acceleration time of the step motor SM1 from the first speedto the second speed is 41 ms, as apparent from FIG. 12.

Subsequently, at step 1119, it is decided whether or not the time T₃ haselapsed. No progress is made so long as the time T₃ has not elapsed. Thetime T₃ is a time required for the trailing edge of the sheet paper toreach just before the paper detector 50 from a point of time when thestep motor SM1 starts to be accelerated to the second speed. Note that,in the present embodiment, the time T₃ is set to 85 ms.

At step 1119, when the time T₃ has elapsed, the routine goes to step1120, at which the step motor SM1 is decelerated from the second speedto the first speed. Namely, the feeding speed of the sheet paper isdecelerated from a high speed of 672 mm/sec to the usual speed 133mm/sec. Note that, the deceleration time is the same as the accelerationtime of the step motor SM1 from the first speed to the second speed,i.e., 41 ms.

At step 1121, the flag F₂ is rewritten from "0" to "1", and subsequentlythe routine goes to step 1122, at which it is decided whether the outputsignal from the paper detector 50 is the low level "L" or the high level"H". Namely, at step 1122, it is monitored whether or not the trailingedge of the sheet paper is detected by the paper detector 50.

At step 1122, when the trailing edge of the sheet paper is detected bythe paper detector 50, that is, when the output signal from the paperdetector 50 is changed over from the low level "L" to the high level"H", the routine goes to step 1123, at which the driving operation ofthe step motor SM1 is decelerated and stopped. The deceleration time ofthe step motor SM1 is 40 ms as apparent from FIG. 12, and at this timethe trailing edge of the sheet paper stops at a position away from thedetection portion by the paper detector 50 by only 5.4 mm in the sameway as the case of FIG. 3. Note that, also in the present embodiment,the outer diameter of the blade-equipped roller 26a is 12 mm, and ahorizontal distance from the detection portion by the paper detector 50to the vertical axial line passing through the center of theblade-equipped roller 26a is 2.4 mm. Accordingly, in the same way as thecase of FIG. 3, the trailing edge of the sheet paper is stopped at aposition away from the vertical axial line passing through the center ofthe blade-equipped roller 26a by 3 mm on the ejection direction side(FIG. 13(b)).

Subsequently, when the routine goes to step 1124, it is decided whetheror not the time T₄ has elapsed. No progress is made so long as the timeT₄ has not elapsed. The time T₄ is appropriately selected and set towithin a range of for example 59 through 270 ms. In short, the sheetpaper is stopped only during a period T₄ and at this time, the trailingedge of the sheet paper is directed to the paper bypass passageway 24side by the blade-equipped roller 26a as indicated by a broken line inFIG. 13(b).

At step 1124, when the time T₄ has elapsed, the routine goes to step1125, at which the step motor SM1 is accelerated to the first speed inthe reverse direction, whereby the sheet paper is fed along the paperbypass passageway 24. At this time, the circumferential speed of thedrive rollers 38a, 40a, and 42a is set to 133 mm/sec, and therefore alsothe feeding speed of the sheet paper fed along the paper bypasspassageway 24 becomes 133 mm/sec. When the sheet paper is fed along thepaper bypass passageway 24, the sheet paper immediately passes theposition of deployment of the blade-equipped roller 26a, but thecircumferential speed of the blade-equipped roller 26a is set to 672mm/sec, and therefore the sheet paper fed along the paper bypasspassageway 24 will not receive any resistance from the blade-equippedroller 26a. Note that, as shown in FIG. 12, the acceleration time of thestep motor SM1 in the reverse direction is 40 ms.

Subsequently, when the routine goes to step 1126, it is decided whetheror not the time T₅ has lapsed. No progress is made so long as the timeT₅ has not elapsed. The time T₅ is a time required for the leading edgeof the sheet paper to reach just before the paper feed rollers 58a and58b provided in the paper bypass passageway 24 from a point of time whenthe sheet paper starts to be fed toward the paper bypass passageway 24.Note that, in the present embodiment, T₅ is 100 ms.

At step 1126, when the time T₅ has elapsed, the routine goes to step1127, at which the step motor SM2 is accelerated to the first speed inthe reverse direction, and at this time, the circumferential speed ofthe drive rollers 58a, 46a, and 48a is set to 133 mm/sec. Accordingly,the sheet paper fed at the feeding speed of 133 mm/sec can be smoothlyaccepted by the paper feed rollers 48a and 48b installed in the paperbypass passageway 24 (FIG. 13(c)).

Subsequently, when the routine goes to step 1128, it is decided whetheror not the time T₆ has elapsed. No progress is made so long as the timeT₆ has not elapsed. The time T₆ is a time appropriately set from a pointof time when the step motor SM2 starts to be accelerated to the secondspeed in the reverse direction and is set to for example 200 ms in thepresent embodiment.

At step 1128, when the time T₆ has elapsed, the routine goes to step1129, at which the step motors SM1 and SM2 are accelerated from thefirst speed to the second speed in the reverse direction. At this time,the circumferential speed of the drive rollers 38a, 40a, and 42ainstalled in the paper eject passageway 22 and the drive rollers 58a,46a, and 48a installed in the paper bypass passageway 24 is set to 672mm/sec. Accordingly, the sheet paper is fed at a high speed of 672mm/sec along the paper bypass passageway 24 toward a pair of registerrollers 34a and 34b.

Subsequently, at step 1130, it is decided whether or not the time T₇ haselapsed. No progress is made so long as the time T₇ has not elapsed. Thetime T₇ is a time required for the trailing edge of the sheet paper toleave from a pair of paper feed rollers 38a and 38b from a point of timewhen the step motors SM1 and SM2 are accelerated from the first speed tothe second speed.

At step 1130, when the time T₇ has elapsed, the routine goes to step1131, at which the step motor SM1 is decelerated and stopped. At thistime, as shown in FIG. 13(d), the sheet paper is completely removed fromthe paper eject passageway 22, and therefore a state of readiness foraccepting the second sheet paper is entered.

Subsequently, at step 1132, it is decided whether or not the time T₈ haselapsed. No progress is made so long as the time T₈ has not elapsed. Thetime T₈ is a time required for the leading edge of the sheet paper toreach just before a pair of register rollers 34a and 34b from a point oftime when the step motor SM1 is decelerated from the second speed to thefirst speed.

At step 1132, when the time T₈ has elapsed, the routine goes to step1133, at which the step motor SM2 is decelerated and stopped.

Subsequently, at step 1134, it is decided whether or not the time T₉ haselapsed. No progress is made so long as the time T₉ has not elapsed. Thetime T₉ is a time required for the leading edge of the sheet paper to beintroduced into the recording unit 14 by a pair of register rollers 34aand 34b from a point of time when the step motor SM2 is decelerated.

At step 1134, when the time T₉ has elapsed, the routine goes to step1135, at which the step motor SM2 is accelerated in the reversedirection.

Subsequently, at step 1136, it is decided whether or not the time T₁₀has elapsed. No progress is made so long as the time T₁₀ has notelapsed. The time T₁₀ is a time required for the trailing edge of thesheet paper to leave from a pair of paper feed rollers 48a and 48b froma point of time when the step motor SM2 is accelerated at step 1135.

At step 1136, when the time T₁₀ has elapsed, the routine goes to step1137, at which the step motor SM2 is decelerated and stopped.

In the same way as in FIG. 2, also in FIG. 9, reference symbol P1indicates the sheet paper initially introduced into the recording unit14 at the time of two-sided printing, and reference symbol P2 indicatesa second sheet paper introduced into the recording unit 14 during aperiod when the sheet paper P1 is fed from the paper eject passageway 22toward the paper bypass passageway 24. In the above-mentioned secondembodiment, when the first sheet paper P1 is fed from the paper ejectpassageway 22 to the paper bypass passageway 24, the feeding speedthereof is partially high (627 mm/sec), and therefore it is possible tomake the interval between the sheet paper P1 and sheet paper P2 narrowerin comparison with that in the first embodiment, and therefore theamount of printing processing at the printing unit 14 is increasedcompared with the first embodiment.

Subsequently, the routine goes to step 1138, at which the flag F₂ isrewritten from "1" to "0". Subsequently, at step 1139, the count valueof the counter C is counted up exactly by +1, and at step 1140, it isdecided whether or not the count value of the counter C is equal to "2".When C is not equal to 2, the routine is once ended.

Thereafter, when the second sheet paper (P2) is returned to a pair ofregister rollers 34a and 34b for the two-sided printing in the samemode, the value of the counter C has been changed to "2", and thereforethe routine goes to step 1141, at which the flag F₁ is rewritten from"0" to "1", and at step 1142, also the flag F₃ is rewritten from "0" to"1". Subsequently, after the counter C is reset at step 1143, it isreturned to step 1101.

At this time, since F₁ =1, the routine goes from step 1101 to step 1103,at which the paper ejection actuation (steps 1104 through 1112) at thetime of one-sided printing is performed, whereby the first sheet paper(P1), that is, the sheet paper subjected to the two-sided printing, isejected onto the paper receiving holder 16. On the other hand, the flagF₃ is made equal to 1 at this time, and therefore the routine goes fromstep 1112 to step 1144, at which the count value of the counter C iscounted up exactly by +1. At step 1145, it is decided whether or not thecount value of the counter C is equal to "2". When C is not equal to"2", the routine is once ended.

Subsequently, a similar paper ejection actuation is repeated (steps 1104through 1112) for ejecting the second sheet paper (P2) onto the paperreceiving holder 16. At this time, the value of the counter C has beenbrought to "2", and therefore the routine goes to step 1140, at whichthe flag F₁ is rewritten from "1" to "0", and at step 1141, also theflag F₃ is rewritten from "1" to "0". Subsequently, after the counter Cis reset at step 1137, the routine is once ended.

Further, when the two-sided printing is carried out also with respect tothe third and fourth sheet papers, the feeding of these sheet papers canbe carried out in the same mode.

FIG. 14 indicates a modified embodiment of the above-mentioned secondembodiment. In this modified embodiment, a pair of paper feed rollers38a and 38b installed in the paper eject passageway 22 are directed sothat the tangential line defined therebetween goes toward the paperbypass passageway 24. According to such an arrangement, when the sheetpaper is brought to a stopped state so as to be fed from the paper ejectpassageway 22 toward the paper bypass passageway 24 (step 1124), it ispossible to more smoothly direct the trailing edge of the sheet paper tothe paper bypass passageway 24 side.

FIG. 15 indicates another modified embodiment of the above-mentionedsecond embodiment. In this modified embodiment, the upper guide plate 60forming the paper eject passageway 22 is arranged close to theblade-equipped roller 26a, and therefore when the sheet paper ejectedfrom the heat fixing unit 36 passes between the blade-equipped roller26a and the upper guide plate paper 60, a tension is given to the sheetpaper. This is because the circumferential speed of the blade-equippedroller 26a is set to 627 mm/sec while the sheet paper is fed at afeeding speed of 133 mm/sec. Immediately after the sheet paper isejected from the heat fixing unit 36, wrinkles frequently occur in thesheet paper, or the sheet paper is bent or deformed. In this case, whenthe sheet paper is returned to the recording unit 14 for the two-sidedprinting, the transfer of the charged toner to such a sheet paper is notcarried out well in certain cases. In the modified embodiment shown inFIG. 15, the tension is given to the sheet paper ejected from the heatfixing unit 36, and therefore wrinkles or bending or deformationoccurred there can be removed.

We claim:
 1. A paper feeder installed in a recording apparatus so as to be able to selectively perform two-sided recording on a sheet paper, said paper feeder comprising:paper supply passageway means for supplying the sheet paper to a recording unit of the recording apparatus; paper eject passageway means for ejecting from said recording unit the sheet paper on which the recording is performed at the recording unit of the recording apparatus; paper bypass passageway means which extends between the paper supply passageway means and the paper eject passageway means; paper switching means provided at a branched portion between the paper eject passageway means and the paper bypass passageway means; and paper feed roller means which can be driven to rotate in forward and reverse directions, provided in said paper eject passageway means whereby the sheet paper can be fed in forward and reverse directions along the paper eject passageway means, wherein the paper feed roller means is arranged on the downstream side of the paper switching means in the ejection direction of the sheet paper; and wherein after the sheet paper on which recording has been performed on one side at the recording unit of the recording apparatus is once fed along the paper eject passageway means by the forward direction driving operation of the paper feed roller means, the sheet paper is fed to the paper bypass passage means through the paper bypass switching means by the reverse direction driving operation of said paper feed roller means, whereby the reversal of said sheet paper is carried out, wherein a portion of said paper eject passageway means positioned on the downstream side of said branched portion and adjacent thereto is arranged so as to form a continuous straight passageway together with a portion of said paper bypass passageway means adjacent to said branched portion; and wherein another portion of said paper eject passageway means positioned on the upstream side of said branched portion and adjacent thereto is angularly arranged with respect to the portion of said paper eject passageway means positioned on the downstream side of said branched portion, and wherein the paper switching means comprises a blade-equipped roller element arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means; and the circumferential speed of the blade-equipped roller element is greater then the normal feeding speed of the sheet paper.
 2. A paper feeder as set forth in claim 1, wherein control for change-over of the paper feed roller means from the forward direction driving operation to the reverse direction driving operation is carried out based on the detection of the passing of the sheet paper at an appropriate position of the paper eject passageway means by a sheet paper detection means installed at said appropriate position.
 3. A paper feeder as set forth in claim 1, wherein the forward direction driving operation of the paper feed roller means is carried out based on the detection of the passing of the sheet paper at an appropriate position of the paper eject passageway means by the sheet paper detection means installed at said appropriate position.
 4. A paper feeder as set forth in claim 1, wherein the reverse direction driving operation of the paper feed roller means is carried out based on the detection of the passing of the sheet paper at the appropriate position of the paper eject passageway means by the sheet paper detection means installed at said appropriate position.
 5. A paper feeder as set forth in claim 1, wherein the paper switching means comprises a roller assembly, which roller assembly includes a blade-equipped roller element, arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means, and two side roller elements which are engaged with the blade-equipped roller element and are arranged on respective sides of said paper eject passageway means and said paper bypass passageway means; and wherein the rotation direction of the blade-equipped roller element is reversed with respect to the rotation direction of the side roller elements, the circumferential speed of the three roller elements being substantially equal to the normal feeding speed of the sheet paper.
 6. A paper feeder as set forth in claim 1, wherein the paper switching means comprises a blade-equipped roller element arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means; and the circumferential speed of the blade-equipped roller element is greater than the normal feeding speed of the sheet paper.
 7. A paper feeder as set forth in claim 1, wherein the feeding direction of said sheet paper is directed toward said paper bypass passageway means when the sheet paper is fed to the paper bypass passageway means by the paper feed roller means by its reverse direction driving operation.
 8. A paper feeder installed in a recording apparatus so as to be able to selectively perform two-sided recording on a sheet paper, said paper feeder comprising:paper supply passageway means for supplying the sheet paper to a recording unit of the recording apparatus; paper eject passageway means for ejecting from said recording unit the sheet paper on which the recording is performed at the recording unit of the recording apparatus; paper bypass passageway means which extends between the paper supply passageway means and the paper eject passageway means; paper switching means provided at a branched portion between the paper eject passageway means and the paper bypass passageway means; paper feed roller means which can be driven to rotate in forward and reverse directions whereby the sheet paper can be fed along the paper eject passageway means in two directions provided in the paper eject passageway means, wherein said paper feed roller means is arranged on the downstream side of the paper switching means in the ejection direction of the sheet paper, wherein when the sheet paper on which recording has been performed on one side at the recording unit of the recording apparatus at the time of two-sided printing is once fed along the paper eject passageway means by the forward direction driving operation of the paper feed roller means, it is fed through the paper switching means to the paper bypass passageway means by the reverse direction driving operation of said paper feed roller means, whereby the reversal of said sheet paper is carried out, and the reverse direction driving operation of the paper feed roller means is controlled so that the feeding speed of said sheet paper is higher than the usual feeding speed thereof over at least a part of a period where the sheet paper is fed to the paper bypass passageway means through the paper switching means by the reverse direction driving operation of the paper feed roller means, and wherein the paper switching means comprises a blade-equipped roller element arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means; and the circumferential speed of the blade-equipped roller element is greater then the normal feeding speed of the sheet paper.
 9. A paper feeder as set forth in claim 8, wherein control for change-over of the paper feed roller means from the forward direction driving operation to the reverse direction driving operation is carried out based on the detection of the passing of the sheet paper at an appropriate position of the paper eject passageway means by a sheet paper detection means installed at said appropriate position.
 10. A paper feeder as set forth in claim 8, wherein the forward direction driving operation of the paper feed roller means is controlled so that the feeding speed of said sheet paper becomes higher than the usual feeding speed thereof over at least a part of a period where the sheet paper on which recording has been performed on one side at the recording unit of the recording apparatus at the time of two-sided recording is once fed along the paper eject passageway means by the forward direction driving operation of the paper feed roller means.
 11. A paper feeder as set forth in claim 10, wherein the forward direction driving control of the paper feed roller means is carried out based on the detection of the passing of the sheet paper at an appropriate position of the paper eject passageway means by the sheet paper detection means installed at said appropriate position.
 12. A paper feeder as set forth in claim 10, wherein the paper switching means comprises a roller assembly, which roller assembly includes a blade-equipped roller element, arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means, and two side roller elements which are engaged with the blade-equipped roller element and are arranged on respective sides of said paper eject passageway means and said paper bypass passageway means; and wherein the rotation direction of the blade-equipped roller element is reversed with respect to the rotation direction of the side roller elements, the circumferential speed of these three roller elements being substantially equal to the usual feeding speed of the sheet paper.
 13. A paper feeder as set forth in claim 12, wherein the paper switching means comprises a blade-equipped roller element arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means; and the circumferential speed of the blade-equipped roller element is greater than the usual feeding speed of the sheet paper.
 14. A paper feeder as set forth in claim 8, wherein the reverse direction driving operation of the paper feed roller means is controlled so that the feeding speed of said sheet paper is higher than the usual feeding speed thereof over at least a part of a period where the sheet paper is fed to the paper bypass passageway means through the paper switching means by the reverse direction driving operation of the paper feed roller means.
 15. A paper feeder as set forth in claim 8, wherein the reverse direction driving control of the paper feed roller means is carried out based on the detection of the passing of the sheet paper at the appropriate position of the paper eject passageway means by the sheet paper detection means installed at said appropriate position.
 16. A paper feeder as set forth in claim 8, wherein the paper switching means comprises a blade-equipped roller element arranged at the branched portion between the paper eject passageway means and the paper bypass passageway means; and the circumferential speed of the blade-equipped roller element is greater than the normal feeding speed of the sheet paper.
 17. A paper feeder as set forth in claim 8, wherein the peripheral portion of the blade-equipped roller element is projected into the paper eject passageway means and arranged close to a guide plate element forming said paper eject passageway means, whereby a tension is given to said sheet element when the sheet element passes between the blade-equipped roller element and the guide plate element along the paper eject passageway means. 